An honest admission that you do not know all the answers is a good witness. It demonstrates that you are on a similar level to the other person, trying to figure things out rather than omniscient. Trying to make authoritative explanations when you do not really have answers is both dishonest in itself and gives you a high probability of guessing wrong. Instead, try to find out an answer or suggest places they might try to find an answer. (This might also weed out people who are just asking questions to cause trouble versus someone willing to wait or work for an answer.)

Confidence is based on our relationship with Christ; thus, the basic answer is "I am not sure how it works, but I know that He is good and in control, so I can be confident in that." Not the easiest to explain to someone wanting physical proof, but something you can trust in.